Precise programs of gene expression control many complex biological processes, including cell growth, differentiation, and development. An elucidating of the mechanisms by which the normal controls are subverted, as occurs in various neoplasms and during the development of some birth defects, will therefore require a basic understanding of the regulation of gene expression. The transcription machinery required for the initiation of transcription at RNA polymerase II promoters is quite complex, including specific DNA-binding proteins, which control transcription only at promoters containing their recognition sequences. Expression from the SV40 late promoter is exquisitely regulated during lytic infection of cells with SV40. The cellular transcription factor LSF, which stimulates transcription from the major late SV40 initiation site, binds two distinct promote elements centered at 45 and 265 bp upstream of this late initiation site. The broad goal of these studies is to define the role of LSF in regulation of transcription both during the SV40 lytic cycle and within the mammalian cell. The specific aims that will address this broad goal are: (1) to clone the gene for LSF, and define both the DNA-binding and transcriptional activation regions of the protein; (2) to study in detail the protein-DNA interactions of LSF at its two recognition sites, and to determine whether bending of DNA and/or different conformations of LSF play a role in the differential recognition; (3) to determine the function of the two LSF binding sites within the SV40 promoters; (4) to determine the mechanism by which LSF stimulates transcription; (5) to investigate potential interactions between LSF and transcription factors binding at adjacent or overlapping sequences, and (6) to determine whether LSF activity is regulated by SV40 infection. LSF (a cellular protein) is likely to also play a role in regulating expression of cellular genes. LSF specifically binds sequences adjacent to the serum response element in the c-fos promoter. In addition, multiple sequences within the cell cycle-regulated human thymidylate synthase and thymidine kinase promoters are highly homologous to LSF recognition sites. The cellular function of LSF will be addressed by delineating LSF binding sites within cellular promoters, determining whether LSF activates or represses transcription of these promoters, determining whether LSF activity is regulated by serum, defined growth factors, or during the cell cycle. These studies will help determine if LSF regulates gene expression in response to specific extracellular signals.